For example, an impeller (see Patent Literature 1) used for a centrifugal compressor or the like is a member requiring high hardness and high toughness since the impeller is exposed to a compression medium while constantly rotating and centrifugal force and high pressure act thereon. For this reason, the impeller is subjected to heat treatment in which it is heated to a predetermined temperature to be tempered within a heating furnace (see Patent Literatures 2 and 3) and is then quenched by blowing a fluid such as nitrogen gas, and thereby has a degree of hardness and toughness suitable for required specifications.
In this way, the impeller is subjected to the heat treatment. However, when heaters arranged on a wall portion within the furnace are not installed all around the wall portion during heating of the impeller, the degree of heating differs between parts in which the impeller is close to the heaters and parts in which the impeller is distanced from the heaters, thereby generating a temperature distribution in the impeller. In addition, even when the impeller is quenched by the nitrogen gas, it is difficult to uniformly blow the nitrogen gas and a flow distribution of nitrogen is wholly generated in the impeller. Accordingly, a temperature distribution is also generated in the impeller.
In particular, this situation is easily generated in a large impeller and the impeller has a variation in hardness and toughness due to the temperature distribution during the heat treatment. For this reason, when centrifugal force acts on the impeller by rotation thereof, there is a possibility that the hardness and toughness may differ at every part of the impeller which may cause oval deformation.
Therefore, a conventional heat treatment is performed in a state in which extra thickness is provided in an impeller and then an uneven portion of the extra thickness is removed by machining and the like, in consideration of the above deformation, so as to correspond to a variation in hardness and toughness generated during the heat treatment.
Here, for example, reducing the temperature distribution generated during heating of the impeller by installing a heat shield plate for shielding heat such that a part of the impeller facing a heater is not directly affected by the heater during heating of the impeller or by installing a stirring fan for uniformizing an atmosphere in a furnace and a temperature thereof may be considered.